Potty train a child

I have THREE children to potty train at once, Twin three year old boys and a little two year old girl. It’s seriously mission impossible.
I put undies on them today and I have to ask them like every ten to twenty minutes if they need to go to the potty, and every half an hour I make them go sit on it and usually they’ll pee if they need to. But no BM’s. Today has been the biggest disaster EVER.
Five minutes after they went to the potty two of them dirtied their pants. I went into their room and they were sitting on one of the beds and there was poo everywhere. It was so so so bad. So I put them both in the shower and tried to clean up the mess. But in the meantime the third child dirtied his pants too.
Do you know how much I want to give up on days like this??
It’s horrible….I feel like crying.